1. Field of the Invention
The present invention relates to a magnetic head and particularly to a composite type magnetic head suitable for use in small-sized and thin floppy disc drives (FDD) and the like, in which recording/reproducing and erasing heads are combined together.
2. Description of the Prior Art
There are known composite type magnetic heads which are used to carry out recording and reproducing on magnetic recording media such as floppy discs and the like and in which recording/reproducing and erasing heads are combined together. As the floppy disc drives and the like are being formed into a reduced thickness equal to or smaller than one inch and into a decreased size equal to or smaller than 3.5 inches, such magnetic heads are also required to be reduced in thickness and size.
Such composite type magnetic heads must be manufactured with higher dimensional precision since any manufacturing tolerance in the magnetic heads themselves may cause variability in electromagnetic transducing characteristics of the recording and reproducing process, resulting in limitations to the compatibility with other machines. The magnetic heads are further required to have reduced mass for providing an improved follow-up property relative to the recording face of the floppy disc, but must conflictingly have sufficients strength for any external impact. If the FDD is mounted in a portable computer, its magnetic head should also be improved in magnetic efficiency to reduce the power consumption. From the viewpoint of manufacturing cost, the magnetic heads must be produced through a simplified process. To meet such requirements, the prior art has improved such magnetic heads in various forms.
There are also known so-called bulk type magnetic heads which comprise a ceramic slider slidably contacting the recording face of a magnetic recording medium and a gimbal spring for supporting the slider on a carriage. The interior of the slider includes a composite magnetic core for forming recording/reproducing and erasing gaps, a recording/reproducing coil assembly mounted in the magnetic core, an erasing coil assembly mounted in the magnetic core and a back bar for magnetically closing the open end of the magnetic core.
One of such magnetic heads has been proposed by the applicant and is disclosed in Japanese Patent Laid-Open No. He/5-135323. This is a composite type magnetic head including two back bars which are suitable for improving the magnetic efficiency, reducing the power consumption and decreasing the thickness and size. This magnetic head will now be described in detail with reference to FIGS. 1 to 6.
Referring first to FIG. 1, a magnetic head 100 is assembled on and supported by a gimbal spring 101 which is in turn fixedly mounted on a carriage (not shown). The magnetic head 100 is fixed to the outer face of the gimbal spring 101, with the coil terminals 102 thereof extending through the gimbal spring 101 to the inner face thereof. The gimbal spring 101 also fixedly supports a core shield (not shown) which surrounds the exterior of the magnetic head 100 to block the magnetism. The inner face of the gimbal spring 101 includes a wiring sheet applied thereto for connecting the coil terminals 102 of the magnetic head 100 to an FDD control circuit. The wiring sheet 103 includes an opening 104 formed therethrough, part of which is disposed opposite to the central portion 105 of the magnetic head 100. The central portion 105 receives a pivot fixed to a carriage.
FIGS. 2 and 3 are perspective views of the magnetic head 100. Particularly, FIG. 2 is a perspective view showing parts of the internal members. The magnetic head 100 comprises a slider 106, a magnetic core 107 at least part of which is received in the interior of the slider 106, recording/reproducing and erasing coil assemblies 108, 109 mounted in the magnetic core 107 and back bars 110 for magnetically closing the open end of the magnetic core 107.
The slider 106 is formed as a ceramic unit and includes main and auxiliary sliding faces 111, 112, on the surface of the slider 106 adapted to face a magnetic recording medium, which are formed into lands slightly raised from the surrounding surface. These sliding faces 111 and 112 are adapted to slidably contact the surface of a magnetic recording medium. The main sliding face 111 includes a slit 113 through which the magnetic core 107 is inserted into the slider 106.
FIG. 4 shows the details of the magnetic core 107. The magnetic core 107 is a composite core having recording/reproducing and erasing cores 114, 115. Each of the cores 114 or 115 includes an outer core 114a or 115a and an inner core 114b or 115b. The outer and inner cores 114a, 114b or 115a, 115b are connected to each other through a given gap. These gaps form a recording/reproducing gap 116 in the recording/reproducing core 114 and erasing gaps 117 in the erasing core 115. The inner cores 114b and 115b are bonded to each other through glass or adhesive. With a magnetic head for 3.5 inch-1 MB recording media, the dimensions of the composite magnetic core 107 are set to be about 3.8 mm in length and about 0.3 mm in width (t.sub.c). The width of the recording/reproducing gap 116 is equal to 131.+-.5 .mu.m while the width of the erasing gaps 117 is equal to 71.+-.5 .mu.m. The two erasing gaps 117 are disposed on the opposite sides of the locus of the recording/reproducing gap and spaced away from each other by a spacing equal to 117.+-.5 .mu.m. A glass part of 20-30 .mu.m is provided outside each of the erasing gaps 117.
FIG. 5 is an exploded and perspective view of the magnetic head 100. The slider 106 includes noses 118 and 119 formed therein to extend from the centers of the opposite inner walls to the magnetic core 107 and adapted to position the composite magnetic core 107. Each of the noses 118 and 119 has a step 118a or 119a for receiving the corresponding one of back bars which will be described later. The top of each of the noses 118 and 119 (as viewed in FIG. 5) is lowered slightly from the top of the corresponding side wall in the slider 106 to form a recessed face 118b or 119b. As shown in FIG. 6, a mass of silver paste is applied to a gap between this recessed face 119b and the gimbal spring 101 to bond them together. This prevents any vibration or judder which may be produced between the gimbal spring 101 and the slider 106.
Coils are wound about the outer cores 114a and 115a in the recording/reproducing and erasing cores of the magnetic core 107. In the embodiment of FIG. 5, conductor wires are wound about coil bobbins 120 and 121 to form coils 122 and 123. The coil bobbins 120 and 121 include bores 124 and 125 formed therethrough into which the legs of the outer cores 114a and 115a are inserted to complete the coil assemblies 108 and 109. In the embodiment of FIG. 5, the coil bobbins 120 and 121 are of the same configuration, and injection molded from a fiber reinforcing plastic which mainly contains polyphenylene sulfide. A core chip center line T, which is the center line of the magnetic core 107 in the direction of thickness, passes through the center of the bores 124 and 125. Parallel frame members 128, 129 or 130, 131 are formed on the inner end of each of the coil bobbins 120 or 121 at the opposite sides and disposed symmetrically relative to each other about the core chip center line T. Orthogonal frame members 132 and 133 are also formed on the inner ends of the respective coil bobbins 120 and 121 at the outer sides and extend in a direction perpendicular to the parallel frame members 128, 129 and 130, 131. These orthogonal frame members 132 and 133 are disposed symmetrically relative to each other about the core chip center line T. The opposite ends 132a, 132b or 133a, 133b of each of the orthogonal frame members 132 or 133 extend laterally beyond the parallel frame members 128, 129 or 130, 131.
Each of the orthogonal frame members 132 or 133 includes three terminals 134, 135, 136 or 137, 138, 139 which are insert molded to extend upwardly beyond the orthogonal frame member. The ends of the coils 122 and 123 are preliminarily soldered and connected to the respective terminals 134-139. Among the three terminals of the erasing coil assembly 109, the central terminal 138 is not wound by the coil end. Each of the parallel frame members 128-131 includes a vertically extending ridge 140, 141, 142 or 143 formed thereon at the central and inner face thereof. The ridge is of semi-circular cross-section having a radius of about 0.3 mm and has a sloped top. An opening 144, 145, 146 or 147 is formed between this ridge 140-143 and the corresponding coil end 126 or 127. Such an opening can cause an appropriate urging force to be effectively applied to the back bars which are inserted between the ridges and the legs of the composite magnetic core 107, as will be described later.
The back bars 110a and 110b are adapted to magnetically close the open end of the composite magnetic core 107. The back bars 110a and 110b are formed of a ferrite material similar to that of the composite magnetic core 107. The back bars 110a and 110b have a thickness (B.sub.1, B.sub.2) ranging between about 0.4 mm and about 0.8 mm and a height ranging between about 0.4 mm and 0.8 mm. The length of each back bar is equal to or slightly longer than that of the magnetic core 107. When the back bars 110a and 110b are inserted while being guided along the sloped tops of the ridges 140 to 143, the tops of the ridges will be shaved by the back bars 110a and 110b. Shavings thus produced can escape through the openings 144-147.
The implications of the fact that the ridges 140-143 are shaved by the back bars 110a and 110b on assembling will be described.
The dimensional tolerances in the coil bobbins 120 and 121, back bars 110 and 110b and composite magnetic core 107 are .+-.20 .mu.m, .+-.5 .mu.m and .+-.5 .mu.m, respectively. The fit tolerance of the back bars 110a and 110b obtained by such a combination becomes equal to .+-.30 .mu.m. The gap in the interface between each of the back bars 110a or 110b and the composite magnetic core 107 must be equal to or less than 1 .mu.m to suppress increase of the magnetic resistance. If the fit allowance in each of the back bars 110a or 110b is set to be between 20 .mu.m and 80 .mu.m, therefore, a substantially large distortion will be created in the composite magnetic core 107 when the ridges 140-143 are not shaped with an increased fit allowance. This reduces the magnetic permeability to provide an insufficient flow of magnetic flux, resulting in degradation of the reproducing and writing efficiencies.
If the ridges 140-143 are not shaved, the top ends of the parallel frame members 128-131 will be widened to degrade the adhesion when the back bars 110a and 110b are assembled into the magnetic head.
By shaving the ridges 140-143 when the back bars 110a and 110b are assembled into the magnetic head, no large force will be applied to the composite magnetic core 107, avoiding the above problem. This can cause the back bars 110a and 110b to be brought into intimate contact with the composite magnetic core 107 without any distortion.
In such a manner, the back bars 110a and 110b are temporarily connected to the opposite sides of each leg in the composite magnetic core 107 as shown in FIG. 3 before they are finally adhered to the legs using the silver paste as shown in FIG. 6 the magnetic head of the prior art which did not use a pair of back bars, a single back bar was only press-fitted into the magnetic head on the side of the main sliding face. In such a case, the thickness of the back bar was larger than that of the magnetic core to suppress increase of the magnetic reluctance. To further reduce the magnetic resistance, the use of two back bars has been proposed. However, such back bars are still of rectangular solid form having the same thickness as before the proposal. Such a back bar was located directly on the side of the auxiliary sliding face. In such a case, the back bar on the side of the auxiliary sliding face will reach a position adjacent to the center of the head. If an impact is exerted on the magnetic head in such a position, the back bar will be urged by the gimbal spring 101 under the pivot of the carriage. The back bar may be broken, thus opening the magnetic circuit or damaging the coil below that back bar, or to producing a shortcircuit. To avoid such risks, the recessed face 119b will be formed adjacent the central portion of the gimbal spring 101 pivoted to provide a gap in which the nose 119 on the side of the auxiliary sliding face is applied to the gimbal spring 101. At the same time, the back bar 110b includes a notch 148 for avoiding the step 119a of the nose 119.
In the aforementioned magnetic head of the prior art, the recording/reproducing and erasing coil assemblies 108, 109 are of a configuration symmetrical about the core tip center line T. When such recording/reproducing and erasing coil assemblies 108, 109 are housed within the slider 106, the end portions 132a, 132b and 133a, 133b of the orthogonal frame members 132 and 133 in the recording/reproducing and erasing coil assemblies 108, 109 will extend outwardly beyond the outer periphery of the slider 106, that is, the plane projection line of the slider 106. When the outer peripheral wall of the slider is shielded, the width of the slider 106 including the magnetic head shield in the seek direction cannot be reduced. In addition, the volume occupied by the upper and lower magnetic heads disposed one above another about the magnetic recording medium will be increased. If the end portions of the orthogonal frame members 132 and 133 further extend outwardly beyond the slider 106, then when the terminals of the orthogonal frame members 132 and 133 are to be wound by the conductor wires of the recording and erasing coils 122, 123, the orthogonal frame members 132 and 133 will be flexed to obstruct the connection of the coil wires.
The terminals 134-139 extend upwardly beyond the tops of the orthogonal frame members 132 and 133 in the recording/reproducing and erasing coil assemblies 108, 109. These terminals 134-139 extend through the gimbal spring 101 to the contact patterns of the wiring sheet 103 which are soldered to the terminals. This will increase the thickness of the magnetic head.
When the terminals 134-139 are to be passed through the holes of the gimbal spring 101 on assembling, the terminals 134-139 may damage the coil wires (which are 30-50 .mu.m in diameter and 1-3 .mu.m in film thickness) to disconnect them. This raises a problem in that such disconnections may first come to light in the market.
The formation of notches in the back bars increases the number of manufacturing steps, resulting in increase of the manufacturing cost. Furthermore, a stress tends to concentrate into the back bars to break them, or requiring the troublesome judgment as to whether or not the back bars to be assembled are properly directed. Since the back bars have their increased thickness to increase the strength, the expensive magnetic core which is more expensive than the back bars tends to be broken at its legs (particularly at the central leg) when a relatively large force is applied to the back bars on assembling.